Whitefly Control

EFFECTIVE CONTROL OF WHITEFLY

Encarsia was one of the first biological control agents developed. This minute (< 1mm, 1/25 inch long) parasitic wasp is probably tropical in origin. It does not sting humans.

With the exception of the adult, all stages of Encarsia occur within the whitefly host. Adults are tiny wasps that lay eggs inside 2 week old whitfly scales (second and third whitfly larval stages). Each egg will kill one whitefly scale. Adults lay up to 200 eggs. The parasite then develops inside the whietfly scale, and midway through the development of Encarsia within the whitefly, the scale turns dark. It becomes black for greenhouse whitefly, and transparent brown for sweet potato whitefly.

This occurs after 10 days at normal greenhouse temperatures. Another 10 days is required before adults emerge. Adult Encarsia emerge from the parasitized scale by chewing a hole in the top of the scale.Adults also kill whitefly scales by direct feeding. Otherwise, they feed on honeydew secreted by the whiteflies. Adults can live for 30 days but normally are active for about 10 days. The complete life cycle requires nearly 28 days in commercial greenhouses.

Eretmocerus eremicus is a tiny parasitic wasp (~1 mm in length) that attacks whiteflies and many species of Aphids. Greenhouse whitefly, Sweet potato whitefly, Silverleaf whitefly, Poinsettia whitefly, woolly whitefly, Citrus whitefly and bayberry whitefly. But unlike E. formosa, which lay their eggs in the 2nd through 4th immature whitefly stages, Eretmocerus eremicus females lay their eggs underneath those same stages, with a preference for the 2nd instar stage. The wasps’ larvae which hatch from the eggs begin to enter the host and thus slowly weaken and kill the developing whiteflies from the outside-in.

METHOD OF APPLICATION

Encarsia are sold as black parasitized scales that have been fixed onto cards. Each card contains 150 developeing parasites. A strip of 10 cards contains 1,500 Encarsia whitefly parasites. It is important to hang the cards from lower leaves in the shade, and avoid wetting them while watering. Apply 1-5 Encarsia per 10 square feet or 1-5 per infested plant BI weekly or until control is achived. Double rates are necessary for the sweet potato whitefly. For greenhouse tomatoes or sweet peppers, 1 Encarsia per 4 plants BI weekly. For greenhouse cucumbers use 1 Encarsia per 2 plants BI weekly. Once the percentage of dark-color, parasitized whitefly scales on leaves exceeds 80%, the numbers of Encarsia in the greenhouse should limit further spread of greenhouse whitefly.

Description and Life Cycle of Whitefly

Whiteflies, both adults and eggs, are found on the underside of leaves. Adults are small grey to white winged flies, measuring 1.25 –2mm, and usually just live for a month. Each female adult will lay up to 200 eggs on the underside of leaves, often in neat circles. These eggs are initially white but darken to an almost black color before hatching into a nymph, which will crawl around the leaf surface before settling to feed. The legs of the nymph then degenerate and the nymph becomes an immobile scale, feeding for 2 weeks before pupating. Adults emerge 10 days later and begin feeding immediately. The development from egg to adult can be as rapid as 3 weeks at warm temperatures, but will take much longer at lower temperatures.

How to Know if You Have Whitefly – The Symptoms

The presence of the various species of whitefly is indicated by large amounts of sticky honeydew and sooty molds on the upper surfaces of leaves, and by a yellow mottling where the whitefly have fed. Whitefly can transmit several strains of plant viruses and infested plants suffer from a lack of vigor.

Of the greenhouse crops, tomatoes and cucumbers are usually most severely infested, although this pest will attack many other common glasshouse and houseplants. Also at risk are cabbages, brussel sprouts and other brassicas, rhododendrons and azaleas.

Biological Control & Treatment of Whitefly

The parasitic wasp, encarsia formosa, has been used with great success to control whitefly populations since 1926. Encarsia females lay eggs directly into the immobile whitefly scales which remain white and develop normally until the encarsia pupates, turning the scale black.

Although it is technically a wasp, it is very small and will not sting you.

The adult encarsia will emerge around 10 days later. The presence of encarsia formosa is indicated by the black parasitized whitefly scales alongside the un-parasitized white scales. Leaves containing black scales are introduced to the greenhouse environment, and under ideal conditions the adults, when they emerge, can lay between 12-15 eggs per day. It is important not to introduce an excessive number of encarsia formosa as they depend on the whitefly scales to continue their life cycle.